Metallic Adhesion Forces and Tunneling between Atomically Defined Tip and Sample

Elements of metallic adhesion are discussed in light of issues in non-contact force microscopy. We have measured forces between an atomically defined W(111) tip and a Au(111) sample in ultrahigh vacuum at 150 K. The W tips are manipulated and characterized on an atomic scale both before and after sample approach by field ion microscopy. We observe strong attractive adhesion forces, which turn repulsive upon further approach of the tip towards the Au surface. Unexpected for a metallic system, there is no spontaneous jump to contact, although we find such wetting is inducible with undefined polycrystalline W tips. The force versus tip-sample distance curve shows only modest hysteresis, and the field ion microscopy images reveal an atomically unchanged tip apex. The tunneling current between tip and sample is found to increase exponentially in a regime of strong adhesive forces.